The present invention relates to an electric discharge lamp apparatus used as a light source of an automotive lighting device such as a headlamp.
An electric discharge lamp has a high luminance, high efficiency and long life. However, the light emitted from such a lamp includes ultraviolet rays of wavelengths which are harmful to health and damaging to adjacent components. Therefore, as proposed in Japanese Patent Unexamined Publication No. Hei-2-253554, etc., and as shown in attached FIG. 5, a ZnO film, which cuts off ultraviolet rays in a predetermined wavelength range, may be formed on the surface of a bulb 4 surrounding an arc tube 2 forming a light source body so that ultraviolet rays in harmful wavelength ranges are eliminated from the light emitted from the arc tube 2.
However, these proposals have been limited to electric discharge lamps for indoor use, as opposed to an electric discharge lamp used as a light source of an automotive lighting instrument such as a headlamp, which must be able to withstand harsh environmental conditions. Specifically, there has been a problem that a lamp having a ZnO film formed on the surface of a glass globe thereof cannot withstand harsh operating conditions such as found in a desert, etc. That is, in an environment where water droplets condense and adhere to the surface of an ultraviolet-ray shielding globe, such as when the temperature, humidity or atmospheric pressure repeatedly changes, there is a problem that the ZnO film separates from the globe, whereupon the ultraviolet-ray shielding effect is reduced. There is another problem in that the transmission factor for visible light is reduced when moisture condenses on the globe because the ZnO film becomes dull when damp.